This invention relates to ink jet printing systems, and, more particularly, to a new and improved configuration of a rail assembly for an ink jet print head. The new ink jet print head rail assembly addresses numerous problems of prior print head rail assemblies. The new ink jet print head rail assembly also addresses numerous problems discovered in the design of very large format, graphics quality digital imaging printers that use scanning ink jet print heads.
Ink jet printing involves placing a number of tiny ink droplets formed by one or more ink jets onto particular locations on the printing medium. The ink droplets solidify or dry on the printing medium, forming small dots. Any number of these small dots, when perceived some viewing distance away from the paper, are perceived as a continuous tone visual image. Both text and graphic images may be printed with ink jet printing.
The printed image from an ink jet printer is made up of a grid-like pattern of potential dot locations, called picture elements or "pixels". For many smaller-format documents commonly viewed from 1-6 feet away, the ink jet printing industry has produced printers having a print resolution of between 200 and 300 pixels per inch (40,000-90,000 pixels per square inch) and a maximum media width of 24 inches. The print resolution for other applications may vary as need, and thus for printing a billboard, commonly viewed from hundreds of feet away, the pixel size may be on the order of 6-12 pixels per inch.
Presently there are two primary types of jets which can be used in ink jet printers. Thermal ink jets use a thin-film resistor to vaporize a small portion of ink and create a minute bubble within the ink. The bubble forces a small droplet of ink through the jet nozzle. Piezo-electric jets use a substrate which is electrically pulsed to create a pressure wave which in turn shoots a droplet of ink through the jet nozzle. A method of making a piezo-electric ink jet is taught in Hoisington et al. U.S. Pat. No. 5,265,315, which is incorporated herein by reference.
Ink jet printers may further be classified as "on demand", for which ink droplets are formed only for the particular pixel locations needed, or as "continuous", for which ink droplets are formed at each pixel location, but some droplets are deflected away before they contact the paper. Additionally, the inks used in ink jet printers may vary.
Ink jet printer systems may use one or more of several different types of ink. Some ink jet printers utilize aqueous inks prepared with water or other solvents which are liquid at room temperature but dry after the ink has been applied to the printing medium. Ink jet systems may alternatively use "hot melt" inks, which contain little or no solvent and are solid at room temperature but are applied in a heated liquid state and then effectively frozen onto the paper surface. One such hot melt ink is taught in U.S. patent application Ser. No. L3.09.12-0006 entitled INK COMPOSITIONS by Elwakil, filed on even date herewith and assigned to the assignee of the present invention and expressly incorporated herein by reference. Ink jet systems also may alternatively use semi-liquid or semi-solid inks, which are semi-liquid or semi-solid at room temperature but are liquified when heated. Such non-aqueous inks are generally known as "phase-change" inks. The present invention applies equally to all these various types of ink jet printers, but is particularly contemplated for on demand, piezo-electric, hot melt ink jet printing.
Color ink jet printers typically use the four subtractive primary colors, cyan, yellow, magenta and black ("CYMK"). Color blending of these four ink colors is achieved through two mechanisms. First, the ink jet printer may lay down multiple colors of ink on the same pixel location, thus combining ink colors at that pixel. The particular color combination caused by having multiple ink colors at a particular pixel location may be affected by the order of printing the various colors, as well as the homogeneity (or non-homogeneity) of ink mixing.
Second, when viewed at a distance, the eye will blend colors from adjacent pixel locations. Thus, for instance, a number of exclusively magenta and yellow dots may be laid down in an area of the image, with no pixel location receiving two colors of ink. Rather than perceiving individual magenta and yellow dots, the eye will blend the adjacent dots to perceive an orange image. In practice, ink jet color printers use both ink blending at particular pixel location and perception blending across pixel locations to create various colors and shades. Often a substantial number of the pixels of the image will go without having a dot of ink placed on them. This allows the perceived visual image to have a proper lightness/darkness or value. Through both forms of color blending, ink jet printers using only four colors of ink can visually reproduce full color images.
Ink jet printers generally move a print head containing the ink jets horizontally across the print image, while advancing the paper lengthwise in between successive passes or scans of the print head. To increase the rate of printing, numerous jets per color have been used to create a wider print head swath or "stroke". Prior ink jet color printers have utilized a single head having 64 linearly aligned jets. To print with four (CYMK) colors, four sets of 16 adjacent jets are each supplied with one of the ink colors to print 16 rows of pixels of each color. Each jet is vertically offset one pixel from the adjacent jets. With this previous 64-jet printer, the paper advance is 16 pixels after each scan (i.e., one quarter of the width of the 64 pixel print stroke), such that each scan of the printer head orients a jet of another ink color over each pixel row printed in a prior color.
Ideally, ink jet printing would occur by vertically-aligned (i.e., aligned in the direction of paper travel, perpendicular to the direction of print head travel) printer jets each mounted one pixel beneath the preceding jet. However, present printer head technology limits the minimal spacing between jets. For instance, piezoelectric jets of the type discussed in Hoisington et al. U.S. Pat. No. 5,265,315 are presently limited to approximately 0.027 inches spacing between adjacent jets, or approximately 37 jets per linear inch. Ink reservoir/firing chamber space is presently the critical factor in preventing closer spacing. To attain 37 jets per linear inch spacing, chambers are alternately located above and below the jets. Resolution of 37 dots per inch is quite unsatisfactory in reproducing closely-viewed visual images of sufficient resolution to produce a pleasing visual effect, such as for graphics-quality, large format output.
To achieve a higher print resolution, prior art linear jet arrays have been oriented at an angle in relation to the direction of print head travel, known as the "saber angle". By angling the linear jet array, the vertical spacing between jets becomes smaller, and the resolution of the resulting image is increased.
To meet the minimal required resolution of 300 dots per inch, the line of jets has been angled such that each jet is positioned approximately 1/300th of an inch vertically beneath the preceding jet. It should be noted that the horizontal spacing between jets should be a multiple of the vertical spacing between pixel rows, to aid in developing a grid pattern of pixel locations having matching horizontal and vertical resolutions. Because the vertical spacing between pixel rows is 1/300th of an inch, the horizontal spacing between jets should be a multiple of 1/300ths of an inch. Given the present spacing constraint of 0.027 inches between the jets, 1/300th of an inch vertical spacing leads to a horizontal spacing between jets of 8/300th of an inch. The prior art 8 to 1 ratio of jet spacing provides a saber angle of 7.125.degree..
Prior art scanning print head configurations, with numerous jets per color each mounted one pixel beneath the previous jet and printing in a full swath, predicate what is known as a "banding" problem. One type of banding occurs if the paper advance is not extremely accurate, such that the paper is advanced slightly more or slightly less than the width of the print swath or stroke (i.e., the vertical extent of the line of jets). That is, if the paper advances slightly too far a perceptible blank area will occur in the color pattern at the end of each paper advance, between the printed swaths. Alternatively, if the paper advance is too short, a perceptible darker area will occur in the color pattern at the beginning of each paper advance, where adjoining swaths overlap.
Other causes can further complicate the banding problem. With some printers, the direction that the print head is traveling for any given scan may affect the order that the different colors of ink are laid down on the paper. A different ordering of colors may create a slightly different hue when visually perceived. For instance, if one band is laid down from left to right with magenta over cyan on a significant number of pixels, and the succeeding band is laid down from right to left with cyan over magenta on a significant number of pixels, a slight color difference between the two bands may be visually detectable.
Banding may also be caused in part by the thermal characteristics of the printing scan. The top of the band may be laid down first, on a relatively cool piece of paper, whereas the middle and bottom of the band may be laid down on a paper heated by previous ink dots. This difference in heating can affect the ink flow characteristics and cause a visually perceptible difference between the top and bottom of the band.
Various methods have been attempted to compensate for the above-cited banding problems. For instance, in Hoisington, et al U.S. Pat. No. 5,075,689, banding was addressed by altering the arrangement of print jets out of a linear array. Another approach to banding, taught by Merna, et al. U.S. Pat. No. 5,239,312, involves altering the spacing between jets on a print head. Both of these previous methods involve additional manufacturing costs in aligning the ink jets into a non-uniform pattern.
A third approach to banding is referred to as "multipass" printing. In multipass printing, the print media is advanced at a fractional increment of the vertical swath width, such that two or more jets of the same color pass over a pixel row on subsequent passes. The first jet will only print a portion of the dots on that pixel row, with remaining dots on the pixel row printed on subsequent passes. Multipass printing tends to mask paper advance errors such that they do not show up as discreet artifacts in the print output, but requires significant additional time in printing.
Ink jet printers often have problems in aligning the jets which are not easily correctable through mechanical manipulation of the head. These alignment problems become aggravated as the number of jets increase, as the spacing between the furthest jets increases during replacement of any other components of the print heads, and as the ink delivery and mechanical placement of print heads becomes more complicated. Alignment problems can largely be compensated by adjusting the data which controls the location of jet firing. Calibration techniques can be used to determine what adjustment is necessary.
Because differences in primary ink colors are easily detectable by the user and provide ready demarcation points relative to the print head, it is common to calibrate each of the four colors with respect to each other. For example, a print head may have a cyan set of jets which is nominally 384 pixels horizontally offset from the black set of jets (16 jets of each color times 8 horizontal pixels per jet times 3 color changes). A test pattern may be laid down by the cyan jets followed by test pattern laid down by the black jets. The test patterns may be compared to determine that, in actual operation, the black set of jets horizontally follows the cyan set of jets by 382 pixels. In such a case, the timing of the black set of jets may be adjusted by two pixel locations, so that patterns laid down by the cyan and black jets will better horizontally match each other. Vertical calibration can be carried out in a similar way. Because calibration is an important part of properly aligned printing, the ink jet printing industry continually seeks new and better ways to readily determine what calibration adjustments are needed.
Additional problems with prior ink jet head configurations involve the mounting of the print head for accurate placement and movement across the printed image. The rail structure for the print head must adequately support the print head not only over the entire printed image, but also for any cleaning, maintenance and other auxiliary functions of the print head. It is common to provide a zone, away from the printing medium within which to "park" the print head to perform auxiliary functions. These auxiliary functions may include cutting of the paper, manipulating ink supply, loading of the paper, certain calibration functions and cleaning of the print head. To accommodate the park zone, the support system, or rails, must support the head over a distance greater than the width of the printing medium. For example, printers handing printing medium about 11 inches wide (which accommodates the length of standard 81/2.times.11 paper) may have rails about 17 inches long.
Accurate placement and movement of the print head becomes more and more difficult as the length of the print scan (i.e., the width of the image) increases. Most prior ink jet printers over about 17 inches wide employ either a two-rail structure, or a single-rail and outrigger structure, for head carriage X-directional travel. Both of these techniques provide two separate and independently adjustable support points for the carriage. Multiple support systems were used on wide printers because it was believed that a single rail could not provide adequate support and stability for the print head over a large distance. Multiple support systems were utilized to provide a wider support base for the print head and carriage to lessen the effect of any stability problems, as well as to provide additional strength to lessen rail flexing problems. Vibration problems may occur if the print head undergoes movement with respect to the rail structure. The print head may slightly rotate or shake about an axis parallel to the rails, causing the print head placement with regard to the paper surface to be inaccurate. Alternatively, the print head may slightly rotate or shake from side to side on the rails, perhaps due to the direction of print head travel. Side to side rotation causes the saber angle to slightly change, altering the placement of ink dots.
Dual support systems are not altogether feasible for graphics quality, large format printing because it is difficult to maintain parallelism of the supports across the entire width of the large format media. More particularly, each support introduces positional error, resulting in non-parallel guide paths for the carriage. Further, prior art two-rail systems employ a pair of circular rails, with the print head mounted on a carriage which is in turn mounted on the rails. The carriage is generally supported by circular sets of ball bearings wrapped around each of the circular rails. Non-parallelism of the rails introduces vibration through the ball bearings to the carriage, often causing instantaneous horizontal velocity errors. If the supports are not parallel, the rollers on the carriage will bind or have excess freedom at particular locations along the rails, and cause further stability and vibration problems. If bending of the rails occurs and the railings are not maintained completely straight, errors occur in positioning the print head. Additional problems occur due to the space that the rails take up, interfering with the transfer of electronics and ink from the printer housing to the print head. It will be .appreciated that these problems are magnified as the length of the rail or rails becomes greater, as in large-format printing. Accordingly, a print head configuration is desired which will avoid these various problems.
One mechanism for cleaning the print head involves wiping the print head with blotter paper as described in Spehrley, Jr., et al. U.S. Pat. No. 4,928,120. The Spehrley, Jr. blotter is provided in a replaceable plastic module. The Spehrley Jr. blotter has a top roller for pressing against the print jet orifices and a bottom roller for pressing against the bottom face of the print head when they are being wiped. While this blotter works acceptably, a less expensive method and apparatus for blotting is desired.
Ink jet printers also need a consistent, accurate method to determine when the ink jets should be fired based on the print head's location with respect to the image. Accurate positioning of ink dots on the printed image is necessary for accurate reproduction of the desired image. Prior art printers have optically sensed markings from an encoder strip to determine print head position. The encoder strip markings are intended to be consistently spaced across the travel of the print head. An encoder strip reader produces a signal as the print head changes location across the encoder strip, and the prior art ink jets are fired based directly on the timing of the encoder strip signal. Prior art encoder strips thus provide one way to determine when the ink jets should be fired.
However, various errors occur which prevent the encoder strip marking from corresponding exactly with the position of an ink dot on the image. These errors tend to be exacerbated as the speed of printing and size of output are increased. High-speed, large format printing requires a high degree of accuracy to generate quality graphics, and a more accurate method of determining when to fire the print head based on its location with respect to the image is desired.